Sensor, sensor system, and method for remotely sensing a variable

ABSTRACT

A sensor system for remote detection of a measurable variable includes a sensor, with a first resonator ( 5 ) which has a resonant frequency that is variable under the influence of the measurable variable; an antenna ( 1 ) for sending and receiving a modulated high-frequency signal; a modem ( 2 ) for coupling the first resonator ( 5 ) to the antenna; and a second resonator ( 3 ) that can be excited by a carrier frequency of the high-frequency signal. An interrogation unit generates an inquiry radio signal for exciting the two resonators and interrupts the broadcasting of the inquiry radio signal in order to receive a response radio signal broadcast by the sensor.

[0001] The present invention relates to a sensor for remote detection ofa measurable variable, to a sensor system in which such a sensor isused, and to a method for remote detection of a measurable variable.

PRIOR ART

[0002] The capability of remote interrogation from a sensor is necessaryin many areas of application, especially where it is problematic toestablish a durable physical connection between a sensor and anassociated evaluation unit, by way of which output signals of the sensorcan be transmitted to the evaluation unit. Such connection problemsoccur wherever the sensor is moved relative to the associated evaluationunit, and especially in the case of rotary motions. As an example ofthis, detecting the pressure in an air-filled tire mounted rotatably ona vehicle can be mentioned, or measuring the torque on a rotating shaft.

[0003] These applications require the transmission of output signalsfrom the sensor electromagnetically in the most general sense, in otherwords transmission of radio signals, microwave signals, or lightsignals. One possibility is to equip the sensor element with its ownelectrical power supply, in order to furnish the energy required formeasuring and transmitting the output signals. However, this principlerapidly meets its limits because of the resultant costs (for a battery),the relatively high weight of the sensor unit, and the requisitemaintenance since after a certain time in operation it is necessary toreplace the battery.

[0004] There is accordingly a need to realize the sensor completelypassively, or in other words without its own power supply, in order tocircumvent the problems associated with the battery and to make thesensor smaller, lighter in weight, and less vulnerable.

[0005] One example of a sensor or sensor system that can be remotelyinterrogated electromagnetically is addressed in German Patent DE 19 702768 C1. The sensor known from this reference includes the following: aresonator, which has a resonant frequency that is variable under theinfluence of the measurable variable, an antenna for sending andreceiving a modulated high-frequency signal, and a modem for couplingthe first resonator to the antenna. For remote interrogation of thevariable measured by this known sensor, an inquiry radio signal isbroadcast by an interrogation unit, and this signal includes an inquirycarrier signal at a second frequency that is modulated with an inquirymeasurement signal at a first frequency. The frequency of the inquirycarrier signal is in the microwave frequency range of about 2.4 GHz,while that of the inquiry measurement signal is in the frequency rangefrom 1 to 30 MHz. The inquiry radio signal is received by the antenna ofthe sensor and applied to the modem, whose output frequency spectrumthereupon has one component at the first frequency. The first frequencyis typically in a resonant range of the resonator, so that on receivingthe inquiry radio signal, this resonator is excited to a compulsoryoscillation, whose amplitude depends, among other factors, on thedifference between the first frequency and the resonant frequency of theresonator, which is dependent on the measurable variable. Once theresonator has been excited to oscillate, the modulation of the inquiryradio signal is interrupted, and the pure, unmodulated inquiry carriersignal is broadcast. This signal mixes in the modem of the sensor withthe then-free oscillation of the resonator at its resonant frequency,and a carrier signal modulated with the resonant frequency in this wayis transmitted back as a response radio signal to the interrogationunit. In the interrogation unit, a conclusion can be drawn about thecurrent value of the measurable variable by evaluating the modulation ofthe response radio signal.

[0006] This known sensor can be excited by an inquiry radio signal ofarbitrary carrier frequency, as long as the modulation frequency isclose enough to the resonant frequency of the oscillator. In order toreceive the response radio signal, the inquiry carrier signal must bebroadcast continuously. It is therefore not possible to use the sameantenna for broadcasting the inquiry radio signal and for receiving theresponse radio signal.

ADVANTAGES OF THE INVENTION

[0007] By means of the present invention, a sensor that can beinterrogated remotely, or a sensor system having a plurality of suchsensors, and a method for remote detection of the measurable variableare created, which make a faster interrogation of the measurablevariable possible along with the simultaneous use of a plurality ofsensors in the same spatial region, without the risk of mutualinterference and without the necessity of coordinating the inquiryoperations of the individual sensors.

[0008] These advantages are attained on the one hand by providing thatthe sensor is equipped with a second resonator that can be excited by acarrier frequency of the high-frequency signal. This design of thesensor makes it possible, during a period of time in which the modulatedinquiry radio signal is broadcast by the interrogation unit, both toexcite the first, tunable resonator to oscillate by means of the inquirymeasurement signal and to excite the second resonator to oscillate withthe aid of the inquiry carrier signal. In this way, energy from thecarrier oscillation is stored at the sensor. The consequence is firstthat to generate the response radio signal, it is no longer necessary tosimultaneously transmit the inquiry carrier signal, because the responseradio signal required can be generated by the sensor, by mixing thesignals of the two resonators at the modem. This response radio signalcan be received and evaluated at the interrogation unit, as soon afterthe interruption of the broadcasting of the inquiry radio signal as theechoes thereof have faded.

[0009] Since the presence of the second resonator makes it possible tointerrupt the broadcasting of the inquiry carrier signal when theresponse radio signal is to be received, it is possible to use the sameantenna in the interrogation unit for sending the inquiry radio signaland receiving the response radio signal. It is moreover possible, in anenvironment in which at least one sensor is assigned to each of aplurality of interrogation units, to assign each interrogation unit andthe sensors belonging to it a specific first carrier frequency, whichenables the interrogation units to respond to and ask questions of onlythe sensors assigned to them.

[0010] The sensor is preferably a purely passive element, without itsown power supply. At the modulator, a detector diode (Schottky diode orvariable actuator) is therefore particularly suitable. These components,because their characteristic curve is already highly nonlinear in thevicinity of zero voltage, generate a strong coupling of the variousspectral components of an applied signal and thus promote thedevelopment of differential or summation frequencies.

[0011] As the first resonator, surface wave resonators or quartzoscillators are suitable. Such resonators are not affected directly intheir behavior by the variable to be detected but instead areexpediently used in an oscillator circuit together with a component thatis sensitive to the variable to be detected. This makes it possible touse economical standard components for the resonators.

[0012] As the element sensitive to the measurable variable, a resistorelement, with a resistance that is variable under the influence of themeasurable variable, is preferably used. Preferred measurable variablesare the pressure or the temperature, for example.

[0013] To prevent the response radio signal from being renderedincorrect by the damping of the oscillation of the second oscillator,which oscillator is unaffected by the measurable variable, thisoscillator is expediently constructed in such a way that it has a lesserdamping than the first resonator, so that in the ideal case, even theoscillation of the second resonator can be considered to be constantduring the measurement period.

[0014] As the second resonator, surface wave resonators are particularlysuitable, which are capable in reaction to a first excitationoscillation pulse of generating a delayed output oscillation pulse. Suchresonators can be embodied for instance surface wave filters with afirst pair of electrodes for exciting the surface wave and athree-dimensionally spaced-apart second pair of electrodes for pickingup the surface wave, or as a resonator with a single pair of electrodesthat serves both to excite and to pick up the surface wave; reflectorelectrodes are disposed at a distance from the electrode pair, in orderto reflect the surface wave, propagating in the resonator substrate, tothe electrode pair with a time lag.

[0015] In the remote detection of a measurable variable using a sensorof the type described above, first its two oscillators are excited by aninquiry radio signal, which includes an inquiry carrier signal at asecond frequency that is modulated with an inquiry measurement signal ata first frequency. To perform a remote interrogation, the broadcastingof the total inquiry radio signal, both of the carrier and of themodulation, is interrupted, and a response radio signal broadcast by thesensor is intercepted, which includes a response carrier signal at theresonant frequency of the second oscillator, modulated with a responsemeasurement signal at the resonant frequency of the first oscillator.Since within this time no inquiry carrier signal can be broadcast, thebackground out of which the response radio signal has to be isolated isonly slight, so that slight reception strengths are sufficient forsatisfactory measurement.

[0016] Since within the time when the response radio signal isintercepted no inquiry radio signal can be broadcast, the same antennacan be used for both sending the inquiry radio signal and receiving theresponse radio signal, without the risk of crosstalk from the inquiry tothe reception.

[0017] If the second resonator furnishes an oscillation pulse that isdelayed compared to the excitation, then it is expedient to select thetime interval within which the inquiry radio signal is broadcast asshorter than the time lag of the second resonator. This in fact meansthat the sensor does not begin broadcasting the response radio signaluntil after a delay after the end of the inquiry radio signal.

[0018] This delay is advantageously selected such that echoes of theinquiry radio signal have faded before the response radio signal arrivesat the interrogation unit.

[0019] Further characteristics and advantages of the invention willbecome apparent from the ensuing description of an exemplary embodimentin conjunction with the accompanying drawings.

DRAWINGS

[0020] Shown are:

[0021]FIG. 1, a block diagram of a sensor according to the invention;

[0022]FIG. 2, a block diagram of an interrogation unit for the sensor ofFIG. 1;

[0023]FIG. 3, the course over time of the intensities in the radiosignals at the antenna of the interrogation unit;

[0024]FIGS. 4 and 5, examples for the layout of a surface wave resonatorwhich is suitable as a second resonator for a sensor of the invention;and

[0025]FIG. 6, the course over time of the intensities in the radiosignals at the antenna of the interrogation unit, when a secondresonator of FIG. 4 or 5 is used.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0026] A sensor system of the invention for remote interrogation of ameasurable variable comprises an interrogation unit, as shown in FIG. 2,and one or more sensors, as shown in FIG. 1. An oscillator 13, whichgenerates a signal, here called an inquiry carrier signal, at a carrierfrequency f_(T) in the range of 2.54 GHz, is located in theinterrogation unit. The carrier frequency is preferably intentionallyvariable by a few MHz. A second oscillator 14 generates an inquirymeasurement signal in the form of an oscillation at a frequency f_(M) inthe range from 0 to 80 MHz. If the interrogation unit is used tointerrogate a plurality of sensors, then the measurement frequency f_(M)is expediently also intentionally variable, specifically in incrementsthat correspond to the magnitude of the resonance range of a firstresonator of the sensors, which will be addressed in further detailhereinafter.

[0027] A modulator 15 is connected to the two oscillators 13, 14; itmodulates the inquiry measurement signal up to the inquiry carriersignal and thus generates an inquiry radio signal, which is output to aswitch 12. The switch 12 is under the control of a timer 16, whichconnects a sending and receiving antenna 11 in alternation to the outputof the modulator 15 and the input of a demodulation and measurementcircuit. The modulation performed by the modulator 15 can in particularbe an amplitude modulation or a quadrature modulation; the demodulationthat takes place in the demodulation and measurement circuit iscomplementary to that.

[0028] The inquiry radio signal broadcast by the antenna 11 is receivedby an antenna 1 of the sensor shown in FIG. 1. Connected to the antennais a demodulation diode 2, such as a Schottky or detector diode. Such adiode is distinguished by a characteristic curve that is alreadyessentially parabolic in the vicinity of the coordinate origin and isthus distinguished by a strongly nonlinear behavior, which leads to amixing of the spectral components contained in the inquiry radio signaland thus to the generation of a spectral component with the frequencyf_(M) of the measurement signal at the output of the demodulation diode2. The spectral component at the carrier frequency f_(T) that alsoappears at the output of the demodulation diode 2 serves to excite aresonator 3, here called the second resonator.

[0029] Also connected to the output of the demodulation diode 2 are alow-pass filter 4 and, downstream of the low-pass filter 4, a so-calledfirst resonator 5, which together with an element 6 that is sensitive tothe measurable variable forms an oscillating circuit. The firstresonator 5, just like the second resonator 3, is a commerciallyavailable component, such as a quartz oscillator or a surface waveresonator. Because of the interconnection with the sensitive element 7,the resonant frequency of the first resonator 5 is variable as afunction of the measurable variable.

[0030] The purpose of the low-pass filter 4 is essentially to keepspectral components in the range of the carrier frequency f_(T) awayfrom the first resonator 5, and thus to prevent them from beingdissipated in the first resonator 5. In this way, the low-pass filter 4not only brings about a more-effective excitation of the secondresonator 3, as long as the inquiry radio signal is being received bythe antenna 1, but also, when there is a pause in the inquiry radiosignal, the low-pass filter 4 limits the damping of the second resonator3.

[0031] The sensitive element 6 can be a resistive element, such as atemperature-dependent resistor, if the measurable variable is thetemperature. A resistive element of this kind has an influence on boththe resonant frequency and the time constant of the first resonator 5.It can also be a capacitive element, such as a micromechanical pressuresensor, with two capacitor plates that are movable relative to oneanother as a function of the prevailing pressure. A capacitive elementof this kind essentially influences only the resonant frequency but notthe damping of the first resonator 5.

[0032]FIG. 3 schematically shows the course of the received fieldintensity P at the antenna 11 of the interrogation unit, as a functionof the time t in the course of one interrogation cycle. The receivedfield intensity P is plotted using a logarithmic scale. During a timeperiod t=0 to t=t₁, the inquiry radio signal is broadcast and is thusnecessarily stronger, by orders of magnitude, than the echo signalsthrown back from the vicinity of the interrogation unit, or than anyresponse signal furnished by a sensor.

[0033] At time t₁, the switch 12 connects the antenna 11 to thedemodulation and measurement circuit 17, and the broadcasting of theinquiry radio signal is interrupted. During a brief time period [t₁,t₂], echoes in the inquiry radio signal, which have been thrown back byobstacles at various distances in the vicinity of the antenna 11 arriveat the antenna 11.

[0034] Once these echo signals have faded, only a response radio signalnow arrives at the antenna 11; this signal has been generated in thesensor by mixing of the oscillations of the two resonators 3, 5 in thediode 2 that now functions as a modulator and has been broadcast via theantenna 1. The demodulation and measurement circuit 17 therefore waits,after the switchover of the switch 12, for a predetermined length oftime Δt before it begins to examine the response signal, received fromthe antenna 11, for its frequency and/or damping and thus to extract theinformation it contains about the measurable variable.

[0035] The delay Δt can be fixedly predetermined as a function of thesending and receiving power of the interrogation unit, for example insuch a way that for a given model of interrogation unit, a maximum rangeis determined, from which echo signals can still be detected by theinterrogation unit, and the delay Δt is selected to be at least equal totwice the transit time that corresponds to this range.

[0036] However, since during the Δt the oscillations of the resonators 3and 5 also fade, it is more advantageous for the delay time Δt to beselected as being as short as possible, as a function of the particularenvironment in which the interrogation unit is used, so that for aspecific usage environment, for instance, the maximum distance of apotential echo source from the interrogation unit is determined, and thedelay is selected to be at least equal to twice the signal transit timefrom the sensor element to the interrogation unit, and thus is selectedto be precisely large enough that an echo from that source is notevaluated.

[0037]FIGS. 4 and 5 show two exemplary embodiments of surface waveresonators, which in a preferred further development can be used as afirst resonator 3 of a sensor of the kind shown in FIG. 1.

[0038] The fundamental design of a structure for exciting and picking upsurface waves from a substrate, with the aid of two electrodes 21, 22deposited on its surface, with a plurality of parallel fingers 24meshing with one another in comblike fashion, is known and need not beexplained here in detail.

[0039] The resonator shown in FIG. 4 includes two pairs 25, 26 of suchelectrodes 21, 22, in each of which one pair, 25 or 26, can serve as atransmitter for exciting a surface wave, while the other pair, 26 or 25,can serve as a receiver for picking up the oscillation. Of the twoelectrodes of each pair 25, 26, one, on the side remote from theopposite pair 26, 25, is provided with a reflector structure 23, whichprevents the propagation of the surface wave. The two pairs 25, 26 areseparated from one another by a spacing L, which causes an oscillationexcited by one pair to reach the other pair at a delay τ≈c/L, where itcan be picked up.

[0040] The surface wave resonator shown in FIG. 5 includes only one pair27 of electrodes, with electrodes 21, 22 that each broadcast in bothdirections perpendicular to the electrode fingers 24. At a spacing L/2from the pair 27 of electrodes, reflector structures 23 are provided,which intrinsically reflect a surface wave, transmitted from the pair 27of electrodes, back again. The reflected surface wave thus reaches theelectrode pair 27 again at the same delay τ≈c/L as in the case of FIG. 4and can be picked up there.

[0041]FIG. 6 is a schematic illustration of the course of the receivedfield intensity P at the antenna 11 of the interrogation unit, as afunction of the time t in the course of one interrogation cycle, whichcourse results when a surface wave resonator of the design shown in FIG.4 or 5 is used as the second resonator of the sensor.

[0042] During a time t=t₀ to t=t₁, the inquiry radio signal isbroadcast, just as in the case of FIG. 3. At time t₁, the broadcastingof the inquiry radio signal is interrupted; the received field intensityP at the antenna 11 decreases to the same extent as the echoes, thrownback from the vicinity of the antenna 11, in the inquiry radio signalfade.

[0043] At time t₃=t₀+τ (signal transit times between the interrogationunit and the sensor are ignored), the surface wave, which has beenexcited by the sensor in the second resonator 3 during the reception ofthe inquiry radio signal, begins to reach the pair of electrodes, whereit is picked up, so that from time t₃ on, a modulated response radiosignal is generated at the sensor. Because the length of the secondresonator 3, or the delay τ within this resonator 3, is selected to begreat enough, it is possible, between the fading of the echos at time t₂and the arrival of the response radio signal at time t₃, for there to bea pause in reception, with a negligible received field intensity that isdetectable by the demodulation and measurement circuit of theinterrogation unit and that permits the interrogation unit todistinguish unambiguously between an echo and a response radio signal.At time t₄=t₁+τ, the surface wave oscillation has passed completelythrough the pair of electrodes performing the pickup, and the generationof the response radio signal ceases.

[0044] After a brief further delay, at time t₅, a new operating cycle ofthe interrogation unit of the sensor begins with the renewedbroadcasting of the inquiry radio signal.

[0045] Typically, the measurement frequency of the inquiry measurementsignal is selected such that with it, an excitation of the firstresonator 5 is possible. However, as a consequence of a major change inthe measurable variable, it can happen that the resultant resonantfrequency of the first resonator 5 is varied so sharply that effectiveexcitation of the first resonator at the frequency of the inquirymeasurement signal is no longer possible. In that case, the responseradio signal cannot be modulated, or cannot be modulated with anintensity sufficient to obtain the measurable variable from the signalreceived at the interrogation unit. It is therefore provided in apreferred refinement of the remote interrogation system that thefrequency of the oscillator 14, that is, the measurement frequencyf_(M), is intentionally variable, and that the interrogation stationvaries this frequency f_(M) if an unusable response radio signal isreceived, or in other words if a response radio signal is received whosequality is insufficient to ascertain the measurable variable from it inthe demodulation and measurement circuit.

[0046] Such a change in the measurement frequency can be effectediteratively, beginning at the particular value f_(M)* of the measurementfrequency f_(M) at which a usable response radio signal was lastreceived. One possible procedure for example is that the vicinity ofthis last usable measurement frequency f_(M)* is investigatedprogressively, in alternation above and below the measurement frequencyf_(M)*, for lesser or greater deviations from this measurement frequencyf_(M)*. It is also conceivable, using two previously used measurementfrequencies, to ascertain a trend in the variation of the resonantfrequency of the first resonator 5, and then to search in the directionindicated by this trend over a plurality of increments, before beginningto search in the opposite direction. Which one of these searchstrategies would be more effective can depend on the particular specificusage environment of the system.

[0047] Under the influence of the measurable variable, the firstresonator is tunable in a frequency range whose width is typically 4MHz. The limits of the tuning range are limits that are not exceeded bythe above-explained search method, either.

[0048] Disruptions in receiving the response radio signal can also occurif a plurality of interrogation units that use the same frequenciesinterfere with one another. Given the purposeful construction of asensor system with a plurality of interrogation units, this problem canbe avoided by providing that each interrogation unit and its assignedsensors are each allocated a specific carrier frequency f_(T) that ischaracteristic for the interrogation unit. This assures that eachinterrogation unit will excite only the second resonators 3 of thesensors assigned to it, so that these sensors can generate a responseradio signal to their interrogation unit only whenever they have beenexcited by that interrogation unit. It is true that inquiry radiosignals broadcast by other interrogation units may possibly excite thefirst resonator 5, if the modulation frequency f_(M) of these inquiryradio signals matches the resonant frequency of the resonator 5;however, since the second resonator 3 is not excited, no response radiosignal can arise.

[0049] When the second resonators of FIG. 4 or 5 are used, which do notfurnish the response carrier signal until after a time lag τ,selectivity in the inquiry can additionally be achieved by providingthat sensors with different delay times τ are used. For example, a delayτ can be fixedly assigned to one interrogation unit, so that it will notdetect response radio signals from sensors that have the same carrierfrequency and measurement frequency as the inquiry radio signalbroadcast by them, because their response radio signals do not occur inthe same time slot, dependent on the delay τ, within which theinterrogation unit evaluates the arriving radio signals.

[0050] It is also possible for one interrogation unit to be assigned aplurality of sensors that have the same carrier and measurementfrequencies but different delay times τ. All of these sensors can beexcited with a single pulse of the inquiry radio signal, but theresponse radio signals that they furnish arrive in succession,chronologically separately from one another, at the interrogation unit,so that the demodulation and measurement circuit can assign the variousresponse radio signals to the individually excited sensors, or to thevariables monitored by them, on the basis of the time at which theyarrive.

1. A sensor for remote detection of a measurable variable, having afirst resonator (5), which has a resonant frequency that is variableunder the influence of the measurable variable, having an antenna (1)for sending and receiving a modulated high-frequency signal and a modem(2) for coupling the first resonator (5) to the antenna (1),characterized in that it includes a second resonator (3), which isexcitable by a carrier frequency of the high-frequency signal.
 2. Thesensor of claim 1, characterized in that the modulator (2) is ademodulation diode, such as a Schottky or detector diode.
 3. The sensorof one of the foregoing claims, characterized in that the firstresonator (5) includes a surface wave resonator or a quartz oscillator.4. The sensor of claim 3, characterized in that the first resonator (5)further includes a discrete component (6) that is sensitive to themeasurable variable.
 5. The sensor of one of the foregoing claims,characterized in that the measurable variable is a pressure or atemperature.
 6. The sensor of one of the foregoing claims, characterizedin that the damping of the second resonator (3) is less than that of thefirst resonator (5).
 7. The sensor of one of the foregoing claims,characterized in that the second resonator (3) is a surface waveresonator, which is capable of generating a delayed output oscillationpulse in reaction to an excitation oscillation pulse.
 8. The sensor ofclaim 7, characterized in that the second resonator has twothree-dimensionally spaced-apart pairs (25, 26) of electrodes (21, 22).9. The sensor of claim 7, characterized in that the second resonator (5)has a pair (27) of electrodes (21, 22) for exciting and picking up asurface wave and has reflector electrodes (23) spaced apart from theelectrode pair (27).
 10. A sensor system, having a plurality of sensorsof one of the foregoing claims, and at least one interrogation unit forsending an inquiry radio signal to the sensors and for receiving aresponse radio signal from the sensors, characterized in that the firstresonators (5) each have different resonant frequency ranges.
 11. Thesensor system of claim 10, characterized in that each interrogation unitis assigned a first specific carrier frequency (f_(T)) of the inquiryradio signal and at least one sensor whose second resonator (5) isexcitable selectively by the specific carrier frequency (f_(T)).
 12. Aninterrogation unit for a sensor of one of claims 1-9 or a sensor systemof claim 10 or 11, characterized in that it has oscillators (13, 14) forgenerating an inquiry radio signal, which includes an inquiry carriersignal at a second frequency (f_(T)), which signal is modulated with aninquiry measurement signal at a first frequency (f_(M)), and has acommon antenna (11) for broadcasting the inquiry radio signal andreceiving a response radio signal from one of the sensors, and theinterrogation unit interrupts the broadcasting of the inquiry radiosignal in order to receive the response radio signal.
 13. Theinterrogation unit of claim 12, characterized in that the frequency ofthe inquiry carrier signal (f_(T)) is variable.
 14. A method for remotedetection of a measurable variable using the sensor of one of claims 1-9or the sensor system of claim 10 or 11, in which the two oscillators (3,5) of one sensor are excited by an inquiry radio signal which includesan inquiry carrier signal at a second frequency (f_(T)), which signal ismodulated with an inquiry measurement signal at a first frequency(f_(M)), and a response radio signal, broadcast by the sensor, isreceived and evaluated, which includes a response carrier signal at theresonant frequency of the second oscillator (3), modulated with aresponse measurement signal at the resonant frequency of the firstoscillator (5), and for receiving the answer radio signal, the sendingof the inquiry carrier signal is interrupted.
 12. The method of claim11, characterized in that the inquiry radio signal and the responseradio signal are sent and received via the same antenna (11).
 13. Themethod of claim 11 or 12, characterized in that if no adequate responsemeasurement signal is received, the frequency of the inquiry responsesignal is varied.